different sub-species of the species mentioned have not been systematically
studied and distinguished.

The origin of new species, which is in part the effect of mutability, is,
however, due mainly to natural selection. Mutability provides the new
characters and new elementary species. Natural selection, on the other
hand, decides what is to live and what to die. Mutability seems to be
free, and not restricted to previously determined lines. Selection,
however, may take place along the same main lines in the course of long
geological epochs, thus directing the development of large branches of the
animal and vegetable kingdom. In natural selection it is evident that
nutrition and environment are the main factors. But it is probable that,
while nutrition may be one of the main causes of mutability, environment
may play the chief part in the decisions ascribed to natural selection.
Relations to neighbouring plants and to injurious or useful animals, have
been considered the most important determining factors ever since the time
when Darwin pointed out their prevailing influence.

From this discussion of the main causes of variability we may derive the
proposition that the study of every phenomenon in the field of heredity, of
variability, and of the origin of new species will have to be considered
from two standpoints; on one hand we have the internal causes, on the other
the external ones. Sometimes the first are more easily detected, in other
cases the latter are more accessible to investigation. But the complete
elucidation of any phenomenon of life must always combine the study of the
influence of internal with that of external causes.

III. POLYMORPHIC VARIABILITY IN CEREALS.

One of the propositions of Darwin's theory of the struggle for life